I wear a Garmin, which I mostly like. The HRV measurement is predictive enough to be useful. Two occasions from the past 18 months are worth noting:
1) The measure dropped from green through yellow and into red. It was in yellow 7 days. On the first day it was in red, I was hit with covid symptoms. It stayed in red 2 1/2 weeks before starting to climb again; my symptoms had mostly cleared by the end of the first week, but I still felt sluggish for some time after. The kicker is that I hadn't looked at the HRV at all until sometime during the week of symptoms.
2) I went on a trip to a location +8 hours off my usual timezone, and jet lag symptoms were brutal. HRV dropped into yellow on day one of the trip, and stayed there during the 2 weeks of travel. A couple days after I returned it dropped into red, exactly coinciding with the onset of flu/covid symptoms. Again, it stayed in red throughout the illness, and then as I started to get fit again on the other side it ramped up through yellow and into green.
So even with 20% accuracy (if that is the case), it's a useful data point! What does 90% accuracy give you?